Systems generating an obstacle map on the basis of positional information of points acquired using a sensor are known. Such systems generate a polar coordinate map specifying occupancy of obstacles for each region obtained by dividing the polar coordinate space into a plurality of regions. The systems use a rectangular coordinate map obtained by converting the polar coordinate map into a rectangular coordinate space, as the obstacle map.
When a polar coordinate map is converted into a rectangular coordinate map, each region of the polar coordinate map is deformed, and the resolution is changed. Specifically, in coordinate transformation, occupancy of each of the regions of the rectangular coordinate space is derived using occupancy specified for one or a plurality of regions corresponding thereto in the polar coordinate space. This structure may markedly reduce the occupancy specified in the polar coordinate map due to coordinate transformation. Accordingly, a conventional art has an insufficient reliability of occupancy of the target, such as obstacles.